The present invention relates to thin film heterojunction photovoltaic devices formed from Group I-III-VI compound semiconductors, such as copper indium diselenide, deposited upon a metal back contact and more particularly to a structure of, a method of making, such devices wherein gallium is incorporated at the interface of the back contact and the compound semiconductor.
The background of copper indium diselenide (CIS) cadmium sulfide solar cells is discussed in considerable detail in U.S. Pat. No. 4,335,266 issued to Mickelsen, et al on June 15, 1982, which patent is hereby incorporated by reference for all purposes. As generally discussed in this reference, this type of device is considered by many to be a viable solar cell. Numerous improvements in efficiency of such solar cells are well documented by publications and patents covering such improvements.
U.S. Pat. No. 4,611,091, issued to Choudary, et al on Sept. 9, 1986, covers one such improvement. This patent is hereby incorporated by reference for its teaching of a structure in which a majority of the cadmium sulfide layer used by Mickelsen is replaced by a substantially conducting wide bandgap n-type semiconductor material such as zinc oxide.
U.S. Pat. No. 4,581,108, issued to Kapur, et al on Apr. 8, 1986, is hereby incorporated by reference for its teachings concerning an improved method for forming CIS for use in Mickelsen type solar cells. In this patent, layers of copper and indium are sequentially deposited on a substrate by electrodeposition. This process provides very close control of the total quantities of these two elements and allows precise control of the ratio or relative amounts of the two elements. As taught by this patent and generally understood in the art, close control of this ratio is considered critical to fabrication of CIS films having good electronic properties. This patent also describes a process known as selenization wherein the electrodeposited copper and indium films are heated at 400 degrees C in the presence of H.sub.2 Se gas to complete formation of CIS material.
Many researchers believe that the conversion efficiency of CIS type solar cells can be increased to over fifteen percent if the device band-gap values can be increased to about 1.5eV. For example, the paper entitled "Low Cost Thin Film Chalcopyrite Solar Cells" by Kapur, et al, Conference Record, 18th IEEE Photovoltaic Specialists Conference (1985), pp 1429-1432 suggests that this may be achieved by fabricating single phase CuIn.sub.1-x Ga.sub.x Se.sub.2 films. A related paper, entitled "Low Cost Methods for the Production of Semiconductor Films for CuInSe.sub.2 CdS Solar Cells" by Kapur, et al, Solar Cells, Vol 21, pp 65-72(1987) reports making of such devices by sequential electroplating of copper, indium and gallium to achieve a ratio of copper to indium plus gallium of 0.95 to 0.98. However, no improvement in performance of finished devices was reported.
An adhesion problem has been recognized at the interface between the typical molybdenum substrate and the CIS film. See for example the above referenced 1985 IEEE Kapur, et al paper. See also the final report entitled "High Efficiency Copper Ternary Thin Film Solar Cells" by Kapur, et al, October 19, 1987 reporting on work performed under Contract No. XL-5-05036-1 for the Solar Energy Research Institute. That paper reports that there appears to be a lack of mechanical adhesion at this interface. The lack of adhesion can cause wrinkling and shunting of the CIS film and in some cases the film spontaneously peels from the substrate. The SERI final report discusses an attempt to plate a 200 angstrom thick layer of indium or lead on the molybdenum surface before plating of copper. However, the report indicates that only "some of the devices made with these structures showed response, and the highest V.sub.oc measured in one device was 330 mV" considerably lower than is normally expected from a CIS type solar cell.